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X-CSE-ConnectionGUID: QtaYWujmSze6WTVlPmigeQ== X-CSE-MsgGUID: tXtJiuBdTfWiMjftQg24dA== X-IronPort-AV: E=McAfee;i="6600,9927,11087"; a="13372390" X-IronPort-AV: E=Sophos;i="6.08,198,1712646000"; d="scan'208";a="13372390" Received: from fmviesa009.fm.intel.com ([10.60.135.149]) by orvoesa113.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 May 2024 09:41:25 -0700 X-CSE-ConnectionGUID: G80LEDOmSAylmCLCHSRfGg== X-CSE-MsgGUID: 4cIPuAd3RueYH9hdPiHLSA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.08,198,1712646000"; d="scan'208";a="35577994" Received: from kinlongk-mobl1.amr.corp.intel.com (HELO [10.125.111.96]) ([10.125.111.96]) by fmviesa009-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 29 May 2024 09:41:24 -0700 Message-ID: <961f9533-1e0c-416c-b6b0-d46b97127de2@intel.com> Date: Wed, 29 May 2024 09:41:22 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v10 00/12] LUF(Lazy Unmap Flush) reducing tlb numbers over 90% To: Byungchul Park Cc: "Huang, Ying" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, kernel_team@skhynix.com, akpm@linux-foundation.org, vernhao@tencent.com, mgorman@techsingularity.net, hughd@google.com, willy@infradead.org, david@redhat.com, peterz@infradead.org, luto@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, rjgolo@gmail.com References: <20240510065206.76078-1-byungchul@sk.com> <982317c0-7faa-45f0-82a1-29978c3c9f4d@intel.com> <20240527015732.GA61604@system.software.com> <8734q46jc8.fsf@yhuang6-desk2.ccr.corp.intel.com> <44e4f2fd-e76e-445d-b618-17a6ec692812@intel.com> <20240529050046.GB20307@system.software.com> From: Dave Hansen Content-Language: en-US Autocrypt: addr=dave.hansen@intel.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/28/24 22:00, Byungchul Park wrote: > All the code updating ptes already performs TLB flush needed in a safe > way if it's inevitable e.g. munmap. LUF which controls when to flush in > a higer level than arch code, just leaves stale ro tlb entries that are > currently supposed to be in use. Could you give a scenario that you are > concering? Let's go back this scenario: fd = open("/some/file", O_RDONLY); ptr1 = mmap(-1, size, PROT_READ, ..., fd, ...); foo1 = *ptr1; There's a read-only PTE at 'ptr1'. Right? The page being pointed to is eligible for LUF via the try_to_unmap() paths. In other words, the page might be reclaimed at any time. If it is reclaimed, the PTE will be cleared. Then, the user might do: munmap(ptr1, PAGE_SIZE); Which will _eventually_ wind up in the zap_pte_range() loop. But that loop will only see pte_none(). It doesn't do _anything_ to the 'struct mmu_gather'. The munmap() then lands in tlb_flush_mmu_tlbonly() where it looks at the 'struct mmu_gather': if (!(tlb->freed_tables || tlb->cleared_ptes || tlb->cleared_pmds || tlb->cleared_puds || tlb->cleared_p4ds)) return; But since there were no cleared PTEs (or anything else) during the unmap, this just returns and doesn't flush the TLB. We now have an address space with a stale TLB entry at 'ptr1' and not even a VMA there. There's nothing to stop a new VMA from going in, installing a *new* PTE, but getting data from the stale TLB entry that still hasn't been flushed.